In order to simulate the thermo-mechanical response of fiber-reinforced composite materials, one must first derive constitutive equations to relate the average stress in the composite material to the average strain in the composite material. These composite average constitutive equations are most often developed by employing a micromechanical model that simulates the manner in which the constituent materials (e.g. fiber and matrix) interact to produce the overall thermo-mechanical properties of the composite material. The micromechanical model can be analytical or numerical in nature, but it is generally accepted that the micromechanical model only approximately represents a) the geometry of constituent materials, b) the material properties of the constituent materials and c) the manner in which the constituent materials interact with each other. Given the approximate nature of the micromechanical model, it is also generally accepted that the use of measured properties of homogeneous bulk constituent material do not result in a micromechanical model that predicts accurate composite material properties.
Consequently, micromechanical models use in situ properties of the constituent materials which effectively account for (i.e., counteract) all of the other approximations and uncertainties inherent in the micromechanical model, thus resulting in a micromechanical model that predicts overall composite properties that closely match the measured properties of the same composite material.
The determination of the in situ constituent properties represents a complex mathematical optimization problem where a consistent, physically-admissible set of constituent properties must be determined so as to cause the micromechanical model to predict composite material properties that agree closely with the measured properties of the composite material. The mathematical problem is non-deterministic since there are more constituent properties to be determined than there are experimentally measured properties for the composite material. Consequently, there are an infinite number of solutions to the problem; however, only a very small number of the solutions for the set of in situ constituent properties yield micromechanical models that can be used for both a) accurately predicting the overall properties of the composite material (known as homogenization), and b) accurately predicting the average stress and strain in the various constituent materials from the average stress and strain in the composite material (known as localization). In general, both of these processes (homogenization and localization) must be performed accurately in order to correctly predict the response of composite structures to thermal and mechanical loading.
What is needed is an automatic method to determine in situ constituent properties. What is also needed is a consistent, accurate, and repeatable method to determine in situ constituent properties.